1. Field of the Invention
The present invention pertains to an image reader suitable for reading a bound document, such as a book or magazine.
2. Description of the Related Art
Image readers, called book scanners, have already been commercialized. In these devices, an image sensing unit is arranged above a document table, so as to provide an open space several tens of centimeters in height between the document table and the image sensing unit. When reading a bound document (hereinafter referred to as "book document") such as a bound document file or magazine, a user places the open book document on the document table so that the document surface is facing upward. When the start key is pressed, an illumination lamp is turned ON, and prescanning is executed. Operation settings are set based on the information obtained by the prescanning, and thereafter a main scanning is executed. In the main scanning, the image read is sequentially output to an external device. If the external device is a printer, a copy image is formed on a real-time. The efficiency of multi-page reading operations is increased because the pages of a book document disposed on document table can be turned when using a book scanner. Furthermore, damage to scanned documents is greatly reduced compared arrangements requiring face-down orientation of open book documents. In the following discussion, book documents include filed documents.
Unlike a single sheet document, the document surface (i.e., the surface to be read) of a book document is curved. In other words, depending on the position along the bilateral direction on the book, the height of the document varies. Due to this height differential, distortion occurs in the read image in accordance with the state of curvature and unsharpness of focus. A book scanner measures the curved condition (the height distribution of the surface of the document), and carries out focus adjustment and distortion correction depending on the result of the measurement. For the method to measure the curved condition of the document surface, a method in which the image of an edge surface, which is the part of the book called the `head`, is taken and image analysis is performed (U.S. Pat. No. 5,585,926), as well as a slit-light projection method used in three-dimensional measurement (U.S. Pat. No. 5,084,611) are available.
In pre-scanning, in addition to the measurement of height distribution, the density of the background of the document, the size of the document and the positions of the user's hands that hold the document to maintain the curved condition are also measured. Measurement of the background density is necessary to set the density of the output image, measurement of the document size is necessary for the purpose of automatic paper selection (APS) and automatic magnification selection (AMS). Depending on the positions of the hands, the range of masking so as to eliminate the images of the hands from the read image is set.
However, in a conventional book scanner, the problem arises that where the curvature of the surface of the document is large, there are areas that go out of focus during pre-scanning, so that the accuracy of measurement during pre-scanning regarding the condition of the document, including the background density and size, decreases and the reading operation parameter settings become inappropriate.
In addition, in a conventional book scanner, because the background density, the size of the document, and the height distribution of the document are all measured based on the information taken from an optical image of one wavelength distribution, the measurement accuracy is not optimal for all measured parameters.